It is the last 78 miles of a NASA rover's 154 million-mile journey to Mars that concerns Ravi Prakash the most.

That's because this is the first time that NASA - or anyone else - has ever tried to land something nearly so big as the 1-ton Curiosity rover on Mars, and because so much is riding on this particular mission.

"We've got to go from five times as fast as a speeding bullet - 13,000 mph - all the way to a screeching halt in seven minutes," said Prakash, a Texas City native who now works at NASA's Jet Propulsion Laboratory on the rover's lander team.

Five times the size of the Spirit and Opportunity rovers already on Mars, Curiosity is packed with scientific equipment: HD-resolution cameras that can also capture video; a laser than can ignite a spark on rocks 20 feet away to determine what they're made of; and other high-tech tools including an X-ray diffraction setup, a mass spectrometer, and a gas chromatograph.

With these devices the six-wheeled rover will be able to sample hundreds of layers of sedimentary rock, allowing scientists to understand how the surface of Mars changed over time, and providing a detailed history of the Red Planet and clues to whether life could have flourished there.

But it's got to get there safely at first.

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Curiosity's illustrious pedigree

NASA has successfully landed a handful of rovers on Mars:

Viking 1 and 2 (1975-1982): After a string of failures, these were the first spacecraft to successfully land on the planet and perform their mission. Combined, the Viking orbiters and landers returned 50,000-plus photos and until 2010 Viking 1 held the record (more than six years) for duration of survival.

Pathfinder (1996-1998): This lander, which released its Sojourner rover at the Martian surface, returned more than 17,000 images and more than 15 chemical analyses of rocks and soil and extensive data on winds and other types of weather. The rover survived three months on the planet's surface.

Spirit and Opportunity (2003-present): Extremely successful rovers that searched for and characterized a wide range of rocks and soils that hold clues to past water activity on Mars. Opportunity, still active, holds the record for survival (eight years and counting) on Mars.

— Eric Berger

Adding to the pressure is that NASA is not currently planning or building a next generation rover to go to Mars. More than 200 scientists attended a NASA meeting earlier this year in Houston to discuss plans for follow-up missions, but none has been chosen.

Nail-biting time

So Curiosity, itself a decade in the works, is it for a long time.

"With no sense of how or when we will follow this up, and without knowing which direction our tools and techniques are evolving - yes, landing successfully is a big deal," said Mark Lemmon, a Texas A&M University planetary scientist who will help operate the rover on Mars.

Launched in November, the $2.5 billion rover will reach the upper limit of the Martian atmosphere at 12:30 a.m. CDT on Aug. 6. Almost out of rocket fuel, it will proceed directly to the planet's surface.

Then the nail-biting will begin. The capsule carrying the rover will slow as it falls through the thin Martian atmosphere, and at seven miles above the Red Planet a parachute will deploy. One mile above the surface, and at a speed of 180 mph, the parachute will separate, and thrusters will further slow the descent.

'Sky crane' maneuver

About 12 seconds before touchdown a completely unprecedented maneuver will occur: as part of a "sky crane" maneuver, cables will lower the rover to the ground at a feathery descent of 1.7 mph. Upon reaching the ground the cables will be cut, and the descent stage will fly away.

If all goes well, Curiosity will be ready to roll in the Gale crater, which rests within a deep depression on the surface of Mars.

Millions of simulations

Given that water flows down hill, the low-lying Gale crater is a place where scientists believe they have a good chance of finding evidence of past water on Mars during one of the planet's wetter epochs.

But will the technical descent work? Scientists say they're 90 to 95 percent confident the landing will be successful.

They've run millions of simulations, tested the parachutes in powerful wind tunnels and performed drop tests on Earth.

Nevertheless it's not entirely possible to simulate the real thing, in which the lander will be operating on the basis of its own sensors and software, with the communication lag between Earth and Mars too long for scientists to make adjustments to the landing during the rapid descent.

Some of those automated step-by-step procedures will be the same as those used by the Apollo landers in their return to Earth, Prakash said. But there will be no astronauts aboard this time to make adjustments.

So the hundreds of scientists on the Mars lander team will be watching from Earth, just like the rest of us.